In the modern world, plastics are the material of choice for the manufacture of a seemingly unlimited number of products. These products are produced by a variety of industrial processes, e.g. injection molding, blow molding, extrusion, and 3-D printers. The raw material that is fed into the machines used to produce the final products is a mixture consisting of: polymers (called resin or virgin in the industry) in the form of small beads, colorants and other additives, e.g. UV inhibitors. The colorants and other additives are supplied as masterbatches, which are concentrated mixtures of pigments and/or additives encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape.
Herein the term “masterbatch” is used to refer to a masterbatch that contains pigment, i.e. color masterbatch, and the term “base material” is used to refer to polymers or mixtures of polymers.
Herein the term “screw” is used to refer to a screw, dosing mechanism, auger, belt conveyer, or vibratory mechanism of the dispensing system
In order to dispense the required amount of the additives material—mainly color masterbatch—to be mixed with the base material volumetric or gravimetric feeders are commonly utilized. One or more such feeders are installed on the throat of the plastic processing machine.
The volumetric system releases a pre-defined volume of additive/masterbatch into the mixing machine. The advantage of this system is implementation simplicity by using a feeding screw, where the released volume is calibrated to the screw rotation speed. This method compromises accuracy for simplicity, since the exact weight (calculated to be volume multiplied by density) of the released masterbatch material for the same rotation speed varies with the masterbatch density, granule size and other parameters.
U.S. Pat. Nos. 5,103,401, 6,688,493B2 and 6,966,456B2 describe gravimetric methods. The gravimetric methods add a weighing mechanism with a control system to the feeding screw, and then, periodically the exact weight of the released material is measured. The difference between the actual weight and the set point is used as the error signal for the control electronics. The gravimetric method has much greater accuracy compared to the volumetric method, resulting in saving of masterbatch material. A gravimetric system allows the material to be released exactly in the amount defined by the set point, usually defined in mass per time unit or percent of the base material. A prior art gravimetric system is shown schematically in FIG. 1.
In both the volumetric and gravimetric cases the masterbatch material set point is defined empirically and no actual measurements of the properties of the mixture are made in-line to confirm/adjust it.
Precision color measurement based on optical spectrum is an extremely challenging process, since fractions of percent of calibration accuracy are required in order to achieve color accuracy better than the color resolution of the human eye.
It is therefore an object of the present invention to provide a system for adjusting and controlling the masterbatch release rate according to an in-line measurement of spectral properties of a product to fit a pre-defined spectral signature of a given reference sample.